Mold apparatus for continuous casting

ABSTRACT

Apparatus for continuous casting including an open-ended mold with a first portion adjacent the inlet end having a low heat transfer capacity wherein no significant solidification of metal takes place, a second portion adjacent the first portion adapted to effect initial skin layer solidification and a third portion adjacent the second portion having a plurality of longitudinal grooves in its inner surface to vent gas released upon solidification from the second portion out the open end of the mold and wherein progressive solidification is accomplished to form solid bar stock.

United States Patent Webbere et al. 5] Feb. 15, 1972 [54] MOLD APPARATUSFOR CONTINUOUS 3,210,812 10/1965 Berwick, Jr ..164/282 CASTING 3,286,31011/1966 Dore et a1 ..l64/281 3,304,585 2/1967 Marchlik 164/281 [72]Inventors: Fred J. Webbere, Orchard Lake; Robert 3,438,426 4/1969 Parfit164/282 Williams, Birmingham, both of Wm 3,459,255 8/1969 Kolle..164/273 x [73] Assignee: glenzral Motors Corporation, Detroit, PrimaryExaminer k Spencer Annfar Att0rneySidney Carter and Peter P. Kozak [22]Filed: Feb. 16, 1970 [21] Appl. No.: 11,407 [57] ABSTRACT Apparatus forcontinuous casting including an open-ended Related US. Application Datamold with a first portion adjacent the inlet end having a low heattransfer capacity wherein no significant solidification of [63]Continuation-impart of Ser. No. 827,747, May 26, metal takes place asecond portion adjacent the first portion 1969' adapted to effectinitial skin layer solidification and a third portion adjacent thesecond portion having a plurality of Ion- [52] U.S. Cl ..l64/283, 164/82 gitudinal grooves in its inner Surface to vent gas released upon [51]Int. Cl ..B22d 11/00 lidifi ti f the Second portion out thc Open end fthe Field of Search 138, 28], 283 mold and wherein progressivesolidification is accomplished to form solid bar stock. [56] ReferencesCited 5 Claims, 8 Drawing Figures UNITED STATES PATENTS Behrenudt 1 64/282 MOLD APPARATUS FOR CONTINUOUS CASTING This application is acontinuation-in-part of our application, Ser. No. 827,747 filed May 26,1969 entitled Mold Apparatus for Continuous Casting.

This invention relates to the continuous casting of bars or rods ofvarious shapes and, more particularly, to a mold apparatus forcontinuous casting of bars formed of relatively high melting pointmetals which are unsaturated in carbon, such as steel, in which ventingof the mold is provided for elimination of gases released from the metalupon solidification.

The apparatus shown and described in our application Ser. No. 827,747included, in general, a horizontally disposed open-ended mold associatedwith a molten metal holding vessel or furnace including a low heattransfer first zone immediately adjacent the holding vessel which iseffective to contain and convey the molten metal therethrough withoutappreciable solidification and which is substantially chemically inertto the molten metal, a relatively high heat transfer second zoneadjacent the first zone which effects the initial solidification in theform of a thin skin of solidified metal progressively and coextensively,first at the interface of the first and second zones and progressivelyto the end of the second zone, and a third zone adjacent the second zonewherein the molten metal is further solidified to form a selfsustainingrod which may be progressively withdrawn from the mold by mechanicalmeans. The apparatus also included means to mechanically pull thesolidified rod from the opposite end of the mold continuously butintermittently in predetermined increments and with predetermined timeintervals between the increments of length or segments of movement ofthe rod wherein each increment corresponds in length to the aforesaidsecond or initial solidification zone so that with each incremental pullof the rod the thin solidified skin layer or segment formed in thesecond zone is advanced into the third zone thereby exposing the secondzone to the advance of molten metal from the first zone and theprogressive soliuification of a new skin layer or segment is formed inthe second zone. The time interval or dwell between the pullingintervals is sufficient to permit the forward end of the newly formedskin layer in the second zone to weld to the rod in the third zone, sothat when the rod is again pulled, it will carry with it the newlyformed segment into the third zone in a continuous rod solidificationprocess.

One of the difficulties encountered in a continuous casting system inwhich the mold is attached directly to the casting vessel is the problemof venting small amounts of gas that may be released by the metal duringsolidification. Accumulation of this gas around the outside of the skinof solidified metal causes interference with normal heat transfer to themold wall and, consequently, incomplete or an improper mode ofsolidification occurs. In the instant mold apparatus in whichsolidification takes place coextensive with the second zone andprogressively in a downstream direction from the interface of the firstand second zone and in which the solidified rod is advanced in anintermittent manner, continuous operation is dependent upon welding ofthe forward end of the newly formed skin layer in the second zone to therod in the third zone at the juncture of the second and third zones. Ifthe normal heat transfer characteristics of the second zone areinterrupted by the accumulation of a layer of gas between the thin skinand the high heat transfer mold wall, a weld of sufficient strength willnot form between the skin layer and the rod during the dwell betweenpulling intervals. On advance of the rod, then, the skin layer remainsin the second zone and molten metal issues against the mold wall at thejuncture of the second and third zones resulting in an impropersolidification process. This phenomenon is known as a breakout" and mayonly be cured by stopping the casting process for an extended time toallow for healing of the rupture to take place. Therefore, in order tomaintain a continuous and controlled solidification sequence duringcasting, it is highly desirable to eliminate these gases from withinthe'mold.

Accordingly, it is the principal object of this invention to provide forventing of gases released from the molten metal on solidification insuch a manner that normal heat transfer within the mold is notinterrupted.

It is a further object of this invention to provide venting means withinthe mold which vents accumulated gases in a downstream direction awayfrom the primary solidification region without being in contact withmolten metal during normal operation of the apparatus whereby surfaceimperfections or interruption of the casting process due tosolidification in the vents is avoided.

These and other objects are accomplished by the provision of ahorizontally disposed open-ended mold associated with a molten metalholding vessel or furnace of the type herein previously described inwhich the third zone wherein the molten metal is further solidified toform a self-sustaining rod is provided with a plurality of longitudinalgrooves cut into the inside surface of the zone beginning at thejuncture of zone 2 and 3, downstream from the primary solidificationzone, and extending to the open end of the mold whereby a longitudinalvoid is provided which provides for venting of gas released upon metalsolidification in zone 2 even though the remainder of the cast surfaceis in intimate contact and in heat transfer relationship with the moldwall. Such an arrangement is particularly applicable to the mold hereindescribed wherein the natural venting resulting from thermal contractionof the bar has been offset by tapering of the inside surface of thethird zone of the mold.

Other objects and advantages of the invention will be apparent from thefollowing description, reference being had to the drawings in which:

FIG. 1 is a cross-sectional view of a horizontal continuous castingapparatus;

FIG. 2 is an enlarged view of a portion of the mold shown in FIG. 1;

FIGS. 3-5 are fragmentary cross-sectional views of the mold at variousstages of the casting process;

FIG. 6 is a view of the mold shown in FIG. 2 taken along line 6-6;

FIG. 7 is a diagram showing the internal dimensions of one embodiment ofthe mold; and

FIG. 8 is an enlarged view of a portion of the mold shown in FIG. 1showing another embodiment of the mold.

Referring now to FIG. 1 of the drawings, the molding apparatus of thisinvention consists generally of a molten metal reservoir 10 shown as afragment thereof and a horizontally disposed open-ended mold 12supported adjacent an opening 14 near the base of the reservoir. Thereservoir is of conventional construction including an outer metal shell(not shown) having a lining 16 of suitable refractory material forcontaining molten metal, such as steel. The opening or channel 14 in thereservoir is formed in a frustoconical refractory body 18 cemented tothe lining 16. The refractory reservoir may include heating means (notshown), such as an induction heating coil or resistance heating elementfor maintaining the metal at a desired casting temperature.

Referring to FIG. 2, the mold 12 consists of three distinctly differentportions with different heat transfer characteristics. The first portionimmediately adjacent the reservoir consists of a noule portion 20preferably formed of boron nitride having relatively low heat transfercharacteristics such that it will contain the molten metal thereinwithout any appreciable solidification. The second portion 22 ispositioned immediately adjacent the nozzle 20 and is formed of materialhaving relatively high heat transfer characteristics as, for example,beryllium-copper alloy. The third portion 24 is disposed immediatelyadjacent the second portion 22 and is preferably provided with agraphite liner 26. The third portion preferably has somewhat lower heattransfer characteristics than the second portion and is provided with aplurality of grooves 28 as will be hereinafter fully explained.

The second mold portion 22 as well as the third mold portion 24 are.both provided with coolant passages 30. The

second mold portion is preferably formed of a berylliumcopper alloybecause of its high heat transfer characteristics. As will be explainedhereinafter, the molten metal contacts the surface of the second moldportion 22 only in a transient manner.

The continuous casting of rods'in the apparatus after the castingprocess is started and is in continuous operation is characterizedbasically by the molten metal passing from the reservoir 10 throughthree successive zones in the mold 12. The molten metal is conveyed fromthe reservoir 10 into the first zone without significant solidificationdue to the sufficiently low heat transfer capacity of the zone andwithout exposure to air. As the metal flows into the second zone, a thinskin of solidified metal is progressively formed along the length ofmold portion 22 due to the high heat transfer capacity thereof. The skinlayer is then advanced as a segment or increment into the third zone ormold portion 24 wherein the molten metal is further solidified to form aself-sustaining rod 31 which is mechanically pulled out of the mold bysuitable means such as rollers 32. As the aforesaid skin layer isadvanced from the second zone to the third zone, a second skin layer isformed in the second zone which subsequently welds itself to the rodbeing solidified in the third zone. The second skin layer is advancedinto the third zone as the rod is pulled incrementally whereby acontinuous rod is formed in a continuous but incremental process.

The following detailed explanation will make the nature of the apparatusand the casting process involved more clear. The reservoir 10 isprovided with a suitable quantity of molten metal such as steel so thatits level extends substantially above the mold 12. The molten metaladvances due to gravity into the mold through a boron nitride nozzle 20which constitutes the aforementioned first zone. Since boron nitride ismaterial of relatively low heat conductivity and is not provided withany cooling means, the molten metal does not significantly solidifytherein.

As soon as the molten metal enters the second zone or the mod portion22, an initial circumferential annulus solidifies against the moldsurface portion 22 at the interface 25 of the nozzle 20 and the moldportion 22, as shown in FIG. 3. This occurs because the mold portion 22is formed of material of relatively high heat conductivity and is cooledby means of a suitable coolant, such as water, circulating in thecoolant passages 30 to provide a high heat transfer capacity whereby afilm or skin of metal solidifies on the mold 22 the instant contact ismade. As the molten metal advances into the second zone, a solidifiedskin layer 34, as shown in FIG. 3, forms progressively on the surface ofthe mold portion 22 in a downstream direction.

The skin layer 34 is then advanced as a segment into the third zone orthe mold portion 24 of the mold where further solidification takes placeto form the self-sustaining rod 31.

When the metal has advanced into the third zone, the solidified layer isof substantial thickness and is relatively cool so that no significantgraphite diffusion occurs in consequence of the graphite liner 26provided in the third zone of the mold. The use of graphite isadvantageous because it is relatively soft and self-lubricating and itpermits the solidified bar to be readily drawn through even though minorimperfections have occurred in the surface of the rod duringsolidification thereof. There is no need for fluid lubricants such asthose commonly used in vertical continuous casting.

As the skin layer 34 begins to advance into the third zone, it mustfirst break away or release from the nozzle 20, as shown in FIG. 4, toform a slight space 36 between the skin 34 and nozzle 20. This space isimmediately filled with fresh molten metal flowing from the nozzle toinitiate formation of the new skin layer 38 (FIG. at the interface 25 ofthe nozzle 20 and the mold portion 22 and to closely follow theadvancing skin layer 34 and to progressively form the new skin layer 38.After the layer 34 has reached its full increment of movement, it ispermitted to remain stationary for a time sufficient to permit the newlayer 38. to weld to layer 34 as shown at 40 of FIG. 5.

It is essential to the successful operation of the process that the skinlayer 34 part cleanly from the nozzle 20 and that it remain stationaryin its advanced position for a time sufficient to permit the new skinlayer 38 to weld thereto. If either of these steps is not performedproperly, a break will occur in the successively formed skin layercausing molten metal to break out and issue against the mold portion 22at 40 thereby preventing proper rod solidification.

As the fresh molten metal flowing from the nozzle into zone two contactsthe mold wall of zone two and solidifies, gases absorbed in the moltenmetal are rejected from the solidifying metal. In addition, the freshlyformed skin layer shrinks slightly from the mold wall due to the volumechange involved in the liquid-to-solid transformation. As a result, thereleased gases collect against the mold wall in the form of a thingaseous layer and accumulate downstream toward zone three assolidification progresses. Simultaneously, gas is being rejected fromthe center of the rod 31 in zone three as solidification progressestoward the center of the rod which passes along the liquid-solidinterface 42 of the rod upstream and toward the mold wall finallyaccumulating against the mold wall in the region 40 at the juncture ofzones 2 and 3 (FIG. 5). This combined release of absorbed gases on skinsolidification in zone 2 and interior solidification in zone 3 resultsin a thin layer of gas along the surface of the mold portion or secondzone 22 with a particularly large amount of gas buildup in the region 40at the juncture of zones 2 and 3.

The presence of this gas layer presents a thermal barrier at the surfaceof the high heat transfer second zone resulting in a decrease of heattransfer through the zone and consequently improper skin formation inzone 2. Furthermore, the buildup of gas at the juncture of zones 2 and 3presents a thermal barrier which inhibits the last metal to solidifyduring the dwell period between rod advances from forming a weld ofsufficient strength to advance the freshly formed skin forward at theend of the dwell period. As a result, on advance of the rod in zone 3 arupture occurs at the welding point, the freshly formed skin remains inplace in zone 2, and molten metal issues against the mold wall at thepoint of rupture instead of at the juncture of the first and secondzones as in normal operation. This breakout of metal forces theoperation to be discontinued to allow for the rupture to heal.

This problem of gas accumulation on the mold wall is overcome byproviding a plurality of longitudinal grooves 28 in the inner surface ofzone 3 (FIG. 2) whereby accumulated gases just the heat transfercharacteristics of zone 3.

The grooves being situated entirely within zone 3 and beginning at thejuncture of zones 2 and 3 provide not only effective venting of gasesfrom the region of highest gas buildup as previously described and fromalong the surface of zone 2, but also eliminate the problem associatedwith molten metal solidifying within the grooves and the resultingretardation of forward advance of the rod. That is, since the groovesare entirely within zone 3 and the formation of the initial skin layeroccurs entirely within zone 2, the grooved portion of the mold is onlyin contact with a solidified skin and, consequently, the grooves willnot be closed by molten metal freezing therein. Occasionally, however,during the process a breakout may occur at the juncture of zones 2 and 3in which case a small amount of metal may penetrate and freeze in thegrooves. However, with the grooves being parallel to the axis ofextraction this will not result in high extraction forces since theridges formed on the rod will follow the grooves along the direction ofrod advance and out the open end of the mold.

The size of the grooves may vary according to operating conditions.Satisfactory casting has been experienced with grooves up to 0.030 inchwide and 0.0l5 inch deep with a pressure head of molten steel up to 24inches.

ln practicing the process of this invention in casting round rods, forexample, it is preferable that the inside diameter of the zone 3 portionof the mold vary in accordance with the progressive solidification ofthe rod so that the mold surfaces are continuously in close contact withthe solidifying and shrinking rod to enhance roundness of the rod and toeffect optimum heat transfer between the mold and the solidifying rod.FIG. 7 is a diagrammatic representation showing a preferred design. Thezone 1 which includes the nozzle is shown to be of constant diameteralthough this is not necessary since the metal exists in this zone onlyin molten form. It has been found advantageous that the mold portion 22have a gradually increasing diameter over its length in the direction ofthe outlet end of the mold in order to reduce the stress in the skinlayer during the first critical advance into zone 3. As shown in FIG. 7,a 5-minute taper with a longitudinal axis in the mold wall producesdesired results in the case where the length of the segment 34 is about1 inch. The mold in the third portion 24 is provided with the groovedgraphite liner 26 and is gradually decreased in diameter in the amountof 4% minutes to the longitudinal axis over the first three inchportion. For casting a 1 7/16 inch diameter bar, this mold portioncontinues to decrease in diameter in increments of 2 to 4 inches byabout two-thousandths of an inch as shown in FIG. 7. The purpose of thistaper is to compensate for the fact that the rod is undergoingsubstantial solidification as may be seen from inspection of FIG. 3.Then, for a final 26-inch interval the diameter of the mold remainsconstant. The zone 3 portion of the mold tapers in accordance with theincreased solidification and decreasing temperature to compensate forshrinkage. The effect is to maintain roundness and good heattransmission from the bar to the mold. In the last 26-inch portion ofzone 3 referred to above, the diameter is preferably maintained constantto slow down heat transfer from the bar to the mold. This is desirableto minimize the reheating of the bar surface and, hence, crackingthereof after the bar has emerged from the mold and to reduce frictionalresistance to the moveme t of the bar. As may be seen, the use ofgroovesin the zone 3 portion is particularly applicable to the use of a taperedmold since the taper maintains the mold and the rod in continuouscontact thus removing the gap between the mold and the rod caused byshrinkage of the rod which would otherwise by itself allow for someventing of gas.

It has been found that beneficial effects can be achieved by replacingfrom 1 to 6 inches of the initial portion of the graphite liner 26 witha corresponding metallic liner 44, shown in FIG. 8, having grooves 28aligned with grooves 28 in the graphite liner 26. The metallic liner 44may be heat shrunk into position within the mold and is preferablyformed of molybdenum or a molybdenum alloy containing predominatelymolybdenum to enhance heat transfer and to provide more resistance toerosion wear than afforded by the graphite alone. The grooved metallicliner results in a more positive healing of occasional breaks that mayoccur in the skin adjacent to the juncture of zone 2 and zone 3 andcontributes to improved life of the remaining graphite liner section. Amolybdenum alloy containing 0.5 percent titanium and 0.08 percentzirconium, available commercially under the name TZM, a product ofClimax Molybdenum Company, has been found to be a particularly suitablematerial for forming the grooved sleeve metallic liner.

In accordance with this invention, round bar stock may be cast in sizesof from about 1 to 3 inches in diameter with a roundness variation of 2%percent or less expressed in terms of the difference in major and minordiameters of the rod.

Although the apparatus has been disclosed in terms of a horizontalcasting process, it will be apparent to those skilled in the art thatthe mold apparatus may readily be adapted for vertical casting.

Although this invention has been described in terms of specificexamples, it is to be understood that other forms of the invention maybe readily adapted within the scope of the invention.

What is claimed is:

1. Apparatus for the continuous casting of a metal rod comprising, incombination, a stationary open-ended mold having an inlet end and anoutlet end and a molten metal reservoir associated with said inlet endin sealed fluid flow relationship,

said mold including a first portion, including said inlet end having arelatively low heat transfer capacity disposed adjacent said reservoir,a second portion adjacent said first portion having a relatively highheat transfer capacity, and a third portion adjacent said secondportion,

said first portion having internal dimensions which are less than theinternal dimensions of said second portion and being so arranged thatthe juncture of said first portion and said second portion within themold cavity is defined by a radially extending wall of said firstportion and an axial wall of said second portion,

the heat transfer capacities of said first portion and said secondportion being related so that molten metal flowing through said mold ismaintained in a substantially completely molten state within said firstmold portion, the high heat transfer capacity of said second portion isoperative to form at least a skin layer of solidified metal on thesurface thereof beginning immediately at said juncture and the heattransfer capacity of said third portion is operative to further solidifythe molten metal to form a self-sustaining rod, the portion of saidfirst zone immediately adjacent said second zone being formed of amaterial which is substantially inert to said molten metal and isnonadherent to said skin layer,

said third portion having a plurality of spaced grooves in the innersurface thereof, said grooves being parallel to the longitudinal axis ofsaid mold beginning at the juncture of said second portion and saidthird portion and terminating at said outlet end for venting gasesaccumulated between said skin layer and said surface of said secondportion in a downstream direction away from said second portion and outsaid outlet end, and

means for advancing said rod formed in said stationary moldintermittently in predetermined segments and at predetermined timeintervals.

2. The apparatus of claim 1 wherein the internal dimensions of saidthird portion progressively decrease in a downstream direction for atleast a portion thereof substantially in proportion to the progressivedecrease in the dimensions of the rod due to shrinkage of said rodsolidified in said mold in such a manner that the mold is in snugcontact with said rod to promote rod shape and the transmission of heatfrom said solidified rod to said mold.

3. The apparatus of claim 1 wherein said grooves are about 0.030 inchwide and about 0.0 l 5 inch deep.

4. Apparatus for the continuous casting of a cylindrical metal rodcomprising, in combination, a stationary openended mold having an inletend and an outlet end and a mo]- ten metal reservoir associated withsaid inlet end in sealed fiuid flow relationship,

said mold including a first portion, including said inlet end having arelatively low heat transfer capacity disposed adjacent said reservoir,a second portion adjacent said first portion having a relatively highheat transfer capacity, and a third portion adjacent said secondportion,

said first portion including a refractory nozzle adjacent said secondportion substantially inert to said metal in molten form having aninternal diameter which is less than the internal diameter of saidsecond portion and being so arranged that the juncture of said firstportion and said second portion within the mold cavity is defined by aradially extending wall of said first portion and an axial wall of saidsecond portion,

said third portion including in at least the initial portion thereof ametallic liner adjacent said second portion and a corresponding graphiteliner adjacent said metallic liner and extending to said outlet end,

the heat transfer capacities of said first portion and said secondportion being related so that molten metal flowing through said mold ismaintained in a substantially the internal diameter of said thirdportion progressively decreasing in a downstream direction for at leasta portion thereof substantially in proportion to the progressivedecrease in the diameter of the roddue to shrinkage of said rodsolidified in said mold in such a manner that the mold is in snugcontact with said rod to promote roundness thereof and the transmissionof heat from said solidified rod to said mold,

said third portion having a plurality of spaced grooves in the innersurface thereof, said grooves being parallel to the longitudinal axis ofsaid mold beginning at the juncture of said second portion and saidthird portion and terminating at said outlet end, for venting gasesaccumulated between said skin layer and said surface of said secondportion in a downstream direction away from said second portion and outsaid outlet end, and

means for advancing said rod formed in said stationary moldintermittently in predetermined segments and at predetermined timeintervals.

5. The apparatus of claim 4 wherein said metallic liner is formed of ametal selected from the group consisting of molybdenum and molybdenumalloys containing predominately molybdenum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIUN Petent No.3,642,058 Q Y Dete'd February 15-, 1972 Inyentor(s) Fred J. Webbere, et.a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent'are hereby corrected as shown below:

Cancel the illustrative drawing on the cover sheet and substitute thefollowinq:

Signed. and sealed this 26th day of December 197.2.

.(sEAL) Attest:

EDWARD-M.FLETCHER,JR. i l ROBERT GOTTSCHALK' Attesting Officer v vCommissioner of'Pat ents- F ORM PO-105O (10-69) USCOMM-DC scam-p69 9 U5. GOVERNMENT PRNYING OFFICE 1 969 O356'334, I

1. Apparatus for the continuous casting of a metal rod comprising, incombination, a stationary open-ended mold having an inlet end and anoutlet end and a molten metal reservoir associated with said inlet endin sealed fluid flow relationship, said mold including a first portion,including said inlet end having a relatively low heat transfer capacitydisposed adjacent said reservoir, a second portion adjacent said firstportion having a relatively high heat transfer capacity, and a thirdportion adjacent said second portion, said first portion having internaldimensions which are less than the internal dimensions of said secondportion and being so arranged that the juncture of said first portionand said second portion within the mold cavity is defined by a radiallyextending wall of said first portion and an axial wall of said secondportion, the heat transfer capacities of said first portion and saidsecond portion being related so that molten metal flowing through saidmold is maintained in a substantially completely molten state withinsaid first mold portion, the high heat transfer capacity of said secondportion is operative to form at least a skin layer of solidified metalon the surface thereof beginning immediately at said juncture and theheat transfer capacity of said third portion is operative to furthersolidify the molten metal to form a self-sustaining rod, the portion ofsaid first zone immediately adjacent said second zone being formed of amaterial which is substantially inert to said molten metal and isnonadherent to said skin layer, said third portion having a plurality ofspaced grooves in the inner surface thereof, said grooves being parallelto the longitudinal axis of said mold beginning at the juncture of saidsecond portion and said third portion and terminating at said outlet endfor venting gases accumulated between said skin layer and said surfaceof said second portion in a downstream direction away from said secondportion and out said outlet end, and means for advancing said rod formedin said stationary mold intermittently in predetermined segments and atpredetermined time intervals.
 2. The apparatus of claim 1 wherein theinternal dimensions of said third portion progressively decrease in adownstream direction for at least a portion thereof substantially inproportion to the progressive decrease in the dimensions of the rod dueto shrinkage of said rod solidified in said mold in such a manner thatthe mold is in snug contact with said rod to promote rod shape and thetransmission of heat from said solidified rod to said mold.
 3. Theapparatus of claim 1 wherein said grooves are about 0.030 inch wide andabout 0.015 inch deep.
 4. Apparatus for the continuous casting of acylindrical metal rod comprising, in combination, a stationaryopen-ended mold having an inlet end and an outlet end and a molten metalreservoir associated with said inlet end in sealed fluid flowrelationship, said mold including a first portion, including said inletend having a relatively low heat transfer capacity disposed adjacentsaid reservoir, a second portion adjacent said first portion having arelatively high heat transfer capacity, and a third portion adjacentsaid second portion, said first portion including a refractory nozzleadjacent said second portion substantially inert to said metal in moltenform having an internal diameter which is less than the internaldiameter of said second portion and being so arranged that the junctureof said first portion and said second portion within the mold cavity isdefined by a radially extending wall of said first portion and an axialwall of said second portion, said third portion including in at leastthe initial portion thereof a metallic liner adjacent said secondportion and a corresponding graphite liner adjacent said metallic linerand extending to said outlet end, the heat transfer capacities of saidfirst portion and said second portion being related so that molten metalflowing through said mold is maintained in a substantially completelymolten state within said first mold portion, the high heat transfercapacity of said second portion is operative to form at least a skinlayer of solidified metal on the surface thereof beginning immediatelyat said juncture and the heat transfer capacity of said third portion isoperative to further solidify the molten metal to form a self-sustainingingot, the internal diameter of said third portion progressivelydecreasing in a downstream direction for at least a portion thereofsubstantially in proportion to the progressive decrease in the diameterof the rod due to shrinkage of said rod solidified in said mold in sucha manner that the mold is in snug contact with said rod to promoteroundness thereof and the transmission of heat from said solidified rodto said mold, said third portion having a plurality of spaced grooves inthe inner surface thereof, said grooves being parallel to thelongitudinal axis of said mold beginning at the juncture of saId secondportion and said third portion and terminating at said outlet end, forventing gases accumulated between said skin layer and said surface ofsaid second portion in a downstream direction away from said secondportion and out said outlet end, and means for advancing said rod formedin said stationary mold intermittently in predetermined segments and atpredetermined time intervals.
 5. The apparatus of claim 4 wherein saidmetallic liner is formed of a metal selected from the group consistingof molybdenum and molybdenum alloys containing predominately molybdenum.